Deprecate C# client

[yurishkuro]

@Falco20019 since you're the primary maintainer of this repo, please opine.

All other Jaeger clients are being retired: jaegertracing/jaeger#3362

The notices are up on all other client repos except this one. Changes to README look like this: jaegertracing/jaeger-client-java@49d4013, coupled with changing the repo description like this:

[Falco20019]

Thanks for the trigger. I will also have a look into upgrading to OTEL in our own application. If I find issues with the migration guide, I will update the README to assist users with guiding them over. But looks straight forward so far.